In a fuel pump for sucking a fuel from a fuel tank and supplying the fuel to an engine, plural permanent magnets are positioned along a circumference of its housing, and an armature is set inside the permanent magnets rotatably. In this way, a driving motor is installed in the fuel pump (in JP-B2-7-85642). In the armature, plural magnetic pole coil portions are formed in a direction of revolution. Each of the magnetic pole coil portions is made by winding a coil on a bobbin, which covers a coil core made of a magnetic material, in a distributed winding form. By supplying electricity from a commutator divided into plural segments to the coils respectively, repulsion or attraction between the permanent magnets and the armature is generated. Thus, since torque is generated in the armature, the armature revolves.
To increase the torque generated in the armature, the amount of discharged fuel and pressure of the discharged fuel, it is considered that the number of windings wound on each bobbin is increased by enlarging spaces for windings by enlarging an outer diameter of the armature. However, the fuel pump is enlarged.
Moreover, it is preferable that the fuel pump is made small by restriction of a space to install the fuel pump. For example, when the fuel pump is installed in a flat-shaped fuel tank, it is preferable that an axial length of the fuel pump is made short. However, when the axial length of the fuel pump is made short without enlarging a diameter of the fuel pump, magnetic fluxes passing through the core decrease. In this case, the generated torque drops.